Department of Justice
(DoJ) The mission of the Department of Justice, under the Attorney General of the United States, is to "To enforce the law and defend the interests of the United States according to the law; to ensure public safety against threats foreign and domestic; to provide Federal leadership in preventing and controlling crime; to seek just punishment for those guilty of unlawful behavior; to administer and enforce the Nation's immigration laws fairly and effectively; and to ensure fair and impartial administration of justice for all Americans."

Source:
http://www.usdoj.gov/02organizations/index.html